Field of the Invention
The invention relates to a method for hot-dip coating a metal strip with a metal covering, wherein the metal strip is directed continuously through a melt bath, wherein the thickness of the metal covering which is present on the metal strip when it leaves the melt bath is adjusted by means of a scraping device, and wherein slag which is present on the melt bath is driven away from the metal strip leaving the melt bath by means of a gas flow. Typically, the metal strips which are coated in this manner are hot or cold-rolled steel strips.
The invention also relates to a device for hot-dip coating a metal strip with a metal covering, this device comprising a melt bath, a conveying device for continuously directing the metal strip through the melt bath, a scraping device for adjusting the thickness of the metal covering which is present on the metal strip when it leaves the melt bath and at least one nozzle for producing a gas flow which drives away slag present on the melt bath from the metal strip leaving the melt bath.
Description of Related Art
The continuous hot-dip processing of the type mentioned in the introduction is an industrially established, economically and ecologically advantageous method principle with which flat metal products can be coated with a metal covering, for example, for the purposes of corrosion protection. The hot-dip processing of a previously in-line recrystallisation annealed metal strip with a Zn (hot galvanising) or aluminium alloy covering (hot aluminium coating) is thus highly significant for the production of semi-finished products for metal sheet applications in automotive engineering, household appliance construction and mechanical engineering.
During the continuous hot-dip processing operation, the annealed metal strip is directed through a melt bath, which comprises a melt of the metal which forms the respective covering or the metal alloy which forms the respective covering and is then redirected within the melt bath via a roller system at least once and stabilised in terms of its path until it leaves the melt bath. Excess covering material which is still molten is then scraped away by means of scraping nozzles after leaving the coating bath. The scraping is generally carried out in this instance by blowing by means of a gas flow. However, scraping systems which function in a purely mechanical manner are also used.
During the immersion phase in the coating bath, some of the steel material of the steel strip is inevitably always dissolved in the coating bath. At the same time, the molten coating bath is permanently in direct contact with the ambient air. Both lead to an unavoidable build-up of slag in the melt bath. This slag floats on the metal bath as so-called “upper slag”.
If upper slag is carried along by the metal strip leaving the coating bath, the coating quality can be impaired in a lasting manner by the resulting defect locations. For example, so-called “smearing strips” appear or the strip is damaged by means of impressions when the carried slag accumulates on subsequent rollers and becomes baked on. This sometimes leads to considerable costs owing to subsequent processing and occurrences of devaluation of the coated metal strip. The removal of relatively large chunks of upper slag, so-called “lumps”, can even lead to costly roller damage in the skin pass mill which is generally arranged downstream in-line.
The installation operator is consequently faced with the permanent challenge of preventing the carrying of upper slag by the coated metal strip to the greatest possible extent.
There are known different possibilities by means of which carrying of slag by the metal strip leaving the metal bath is supposed to be prevented.
Manual/mechanical methods should be mentioned here first. In practice, the upper slag is removed from the coating bath at short time intervals by workers using removal tools. This operating method has the disadvantage that the removal of upper slag is carried out in a discontinuous manner and there are consequently always time intervals—even if short—in which upper slag can come into contact with the discharged metal strip in an unimpeded manner. When the upper slag is removed manually from the direct vicinity of the metal strip leaving the melt bath, the quality of the coating can be further impaired by means of excessive agitation of the coating bath and by touching the metal strip with the scraping tool.
There are also known so-called slag removal robots which are driven in a motorised manner and which automatically remove the slag from the respective melt bath. Such slag removal robots imitate manual removal and, owing to structural circumstances, cannot be installed on every hot-dip coating installation.
Also used in practice are so-called mirror rollers which are positioned parallel with the width axis of the discharged metal strip and which remove the slag which comes into contact with them and which remains bonded to the surface thereof from the slag bath. The device described in DE 10 2006 030 914 A1 also belongs to this prior art in which a motor-driven operating means scrapes the upper slag from the coating bath surface with uniform speed. Although the use of motorised mirror rollers or motorised scraping means allows a continuous operating method, moving components are in permanent contact with the coating bath in this instance. The daily industrial routine shows here that the aggressive nature of the molten coating bath produces considerable wear in such moving components. This applies to the coating of a steel strip with an aluminium-based covering (hot-aluminium coating).
A third possibility for keeping the slag away from the metal strip leaving the melt bath involves continuous rotation of the coating bath and the installation of cooling zones by means of which slag formation can be displaced in a selective manner into surface regions of the melt bath remote from the strip path. The effectiveness of these measures can be increased by the flows within the coating bath being directed in such a manner that they act counter to the strip path. Loosened metal strip components are thereby transported away from the metal strip. Methods of this type are described in WO 2009/098362 A1, WO 2009/098363 A1, U.S. Pat. No. 5,084,094 A1, U.S. Pat. No. 6,426,122 B1 and U.S. Pat. No. 6,177,140 B1, respectively. A problem with these methods is that they partially require very complex and costly devices which in many cases cannot be retrofitted in each existing hot-dip coating installation. It has further been found that the required bath flow can be maintained within a daily industrial routine only with great difficulty. Furthermore, in the device required to carry out these methods, many mechanical components come into direct contact with the molten coating bath and are accordingly subjected to a high degree of wear.
When excess covering material is scraped from the metal strip by means of scraping nozzles which are positioned directly above the coating bath surface, high gas pressures and accordingly high flow speeds of the gas flow have the positive side-effect that a partial gas flow which is directed to the coating bath surface presses upper slag away from the metal strip being discharged. Scraping nozzles which achieve this are described, for example, in DE 43 00 868 C1 and DE 42 23 343 C1. However, the removal of the slag from the metal strip leaving the melt bath is carried out in an uncontrolled, rather random manner. With low gas pressures such as those which are adjusted in the case of low strip travel speeds or in the case of high coating thicknesses, the side-effect involving “pressing the slag away from the metal strip leaving the melt bath” does not occur.
From JP 07-145460, it is finally known to arrange a nozzle carrier transversely relative to the metal strip leaving the melt bath and parallel with the surface of the melt bath in such a manner that the gas being discharged from it the slag present on the melt bath is driven laterally with respect to the outer edge of the melt bath by gas flows which act parallel with the strip and tangentially to the surface of the melt bath and which are directed away from each other in the manner of the roof surfaces of an acutely gabled roof. The slag which is accumulated there can then be mechanically removed. However, a disadvantage of this procedure which is closest to the invention is the dead space which is inevitably produced in the region below the nozzle carrier. In this dead space, there may be an accumulation of slag which comes into contact with the strip leaving the melt bath and which leads at that location to significant surface defects at the centre of the strip width. Another disadvantage of this procedure is that the gas flows of the nozzle bar are arranged for the most part with significant spacing from the metal strip and accordingly slag is driven to a location of the surface of the melt bath on which there is no danger at all of penetration of the metal strip with slag. This leads to unnecessary gas consumption.